Temperature probe arrived

Finally, got hold of a temperature probe (PT100 from eBay).

Most conductive materials have properties that change with temperature, materials whose resistance changes reliably with temperature are used as thermistors. PT100 devices uses platinum as that material, and has a very linear response with temperature compared with other devices.

More importantly though, I bought this PT100 probe simply because it’s housed in a nice stainless steel probe!

PT100 will be connected like this:

With the equation:

Vout = Vin R2 / (R1RTD + R2)

The resistance of a PT 100 is given in this PT100 chart, the temperature-to-resistance profile isn’t exactly linear, so the code in the microcontroller must make a conversion

Although as a demonstration, this chart shows the error when trying to linearise the PT100 temperature response curve

Now my PT100 is terminated with three wires, this is to reduce the errors introduced by the resistance of the connecting wires. For example, if it only had one wire, the equivalent circuit would look like this:

The effective resistance of the RTD (resistive temperature device) is therefore the resistance of the RTD plus the resistance of the two connecting wires, skewing the results a bit.

But with three wires connected like this:

The effective circuit looks like:

Note now that the resistance introduced by the wire is symmetrical now, and so does not skew the results as much.

Also, four-wire probes are available:


WordPress won’t let me upload zip files, so I’ll put the code up on GitHub at some point


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